In multi-story buildings such as office buildings, hotels, apartment buildings, etc. a machinery room is normally located on each floor. Since the space occupied by the machinery room represents space unavailable for renting, it is desirable to minimize such space. Because the machinery room usually backs on the elevators, the space required by the elevators can dictate one of the dimensions of the machinery room and the space between floors dictates a second dimension. The air handling unit (AHU) for circulating conditioned air throughout a floor is located in the machinery room on that floor. The noise from the air handling unit, particularly at low frequency, has A become a major concern for building occupants in recent years due to its impact on room sound quality.
To control the noise from AHUs, duct active noise control (ANC) systems are starting to be employed in air distribution systems. An ANC system basically requires the sensing of the noise associated with the fan for distributing the air, producing a noise canceling signal and determining the results of the canceling signal so as to provide a correction signal to the loudspeaker. There is a time delay associated with sensing the noise and producing a canceling signal. This time delay necessary for the canceling to take place equates to the distance in the system required between the reference, or input, noise sensor and the loudspeaker. Additional space is required between the loudspeaker and the error sensor which also equates to a distance in the system. The space limitations in existing buildings severely limits the retrofitting or replacement of existing equipment with equipment using conventional ANC approaches. In new buildings the extra space required by conventional ANC approaches comes at a high price in the reduced rentable space which would result on each floor.
Other than the additional space requirements associated with duct ANC systems, there are conventional design approaches associated with fans that preclude significantly reducing the size of the fan and the fan discharge. Specifically, in conventional ANC designs a further problem is that the input noise sensing microphone cannot be located near the fan discharge where a high level of turbulence prevents accurate sound measurement and thereby compromises noise cancellation. To offset this problem, designers are forced to place the noise sensing microphone far from the discharge, for example three times the blower diameter downstream, so that flow leaving the blower discharge can fully occupy the duct and hence lower the level of turbulence. In the case of top discharge units, a large elbow transition would also be required.